VHDL uses a library system to maintain designs for modification and shared use. VHDL refers to a library by an assigned logical name; the host operating system must translate this logical name into a real directory name and locate it. The current design unit is compiled into the Work library by default; Work is implicitly available to the user with no need to declare it. Similarly, the predefined library STD does not need to be declared before its packages can be accessed via use clauses. The STD library contains the VHDL predefined language environment, including the package STANDARD which contains a set of basic data types and functions and the package TEXTIO which contains some text handling procedures.